1. Technical Field of the Invention
The present invention relates to optimizing motor torque, and particularly to a control system and method for controlling a polyphase motor that provides substantially ripple free torque.
2. Background of the Invention
Electronically switched DC motors are used in many control and regulation applications. Switched DC motors are also used in mass memory drive systems for rotating media, such as hard disks, floppy disks, optical disks, and CD-ROMs, as well as for linear media, such as tape streamers and the like. Commonly these motors are polyphase motors in a xe2x80x9cstarxe2x80x9d configuration. It is quite common for such a motor to have three phase windings connected in a star configuration and defining six different switching phases and P number of poles.
These brushless motors are commonly driven using an integrated circuit whose output stage is represented by a polyphase full-wave bridge circuit. In the case of a three-phase motor the bridge circuit may employ six bipolar (BJT) or field effect transistors (MOS) power transistors. The motor current is linearly controlled through a transconductance loop.
As known in the art, the drive signals applied to the coils of the motor may take on different waveforms, depending upon the system and the desired operation. Two common types of driving signals are linear and digital driving signals. Linear driving signals tend to have waveforms that are fairly continuous in nature, such as a direct current (DC) signal. Digital driving signals tend to have waveforms that are switched on and off over time, such as a digital pulse train. Pulse width modulation (PWM) is one example of a scheme to drive an electric motor using a digital pulse train. For instance, commonly assigned U.S. Pat. No. 4,972,130 issued Nov. 20, 1990 discloses a particular system that uses PWM driving circuits for driving the coils of a motor.
A typical objective in either a linear or a digital motor control system is to establish and maintain the operation of the motor as required for the application. For example, in a disc drive, the rotational speed of a motor may be held substantially constant, for a given load, by applying drive signals that supply a constant current to the coils so as to maintain a somewhat constant torque.
In order to cause the desired torque in the motor, brushless motors typically require a motor controller capable of selectively connecting and disconnecting (i.e., commutating), each of the motor""s coils to and from the driving signals at particular times. Calculating the proper commutation time usually requires determining, or monitoring, the location of the motor""s rotor with regard to the coils. This may be accomplished, for example, by including sensors that relate such information to the motor controller circuit, or by evaluating a bemf signal generated in one or more of the coils within the motor. For sensorless motors, the bemf signal may be fed-back to the motor controller to determine the commutation time along with the difference (i.e., error) between the actual and desired rotational speeds. Such techniques are known to those skilled in the art, and include for instance, the methods and apparatuses disclosed in commonly assigned U.S. Pat. No. 5,317,243 issued May 31, 1994, U.S. Pat. No. 5,306,988 issued Apr. 26, 1994, U.S. Pat. No. 5,223,772 issued Jun. 29, 1993, and U.S. Pat. No. 5,221,881 issued Jun. 22, 1993, each of which is incorporated herein by reference.
Typically, to drive the motor in a given direction, the motor is driven with a current in the direction that provides for a positive total torque. It is known in the art that to achieve maximum efficiency the commutation should be performed when the Bemf on two phases is equal.
Unfortunately, a torque ripple may be introduced into the motor during commutation. Torque ripple can produce jitter in the motor and possibly an accompanying, acoustical noise. Torque ripple can typically be found in both linear and PWM systems because of the torque fluctuations occurring during commutation of phases due to the abrupt decay of the current in one coil and the relatively slower rise of the same in the next energized coil. The effects of torque ripple, such as introducing jitter in the system, are well known to those skilled in the art. For instance, commonly assigned U.S. Pat. No. 5,191,269 issued Mar. 2, 1993, addresses such problems in a linear system by disclosing circuitry that reduces torque ripple in a linearly driven motor.
It is therefore the goal of many systems to maximize the torque, while minimizing the torque ripple. In theory, it is possible to design an optimal sinusoidal (linear) or pseudo-sinusoidal (digital) driving circuit wherein each of the back electromotive force (bemf) phase signals is in phase with its respective driving signal""s current. Assuming that the bemf signals are sinusoidal or nearly sinusoidal signals, the power flow (energy) in such a system would theoretically be a constant, in accordance with the following equation:
xe2x80x83sin2(xcfx89t)+sin2(xcfx89t+120xc2x0)+sin2(xcfx89t+240xc2x0)=1.5
Thus, in principle such a system would yield zero torque ripple.
In practice, however, it is often very difficult to design such a digital system that controls the motor so that each bemf signal for a motor winding is in phase with the corresponding driving signal""s current. Based upon the foregoing, there is a need for a motor controller for driving a polyphase motor so as to yield a substantially optimal torque.
The present invention overcomes the shortcomings in prior systems and thereby satisfies a significant need for a controller for a polyphase motor that substantially optimizes motor torque. The controller includes a memory device having stored therein data representing a predetermined drive signal profile for driving the windings of the motor. A driver circuit applies drive signals to the motor windings based upon the data provided by the memory device. A feedback control loop provides an address to the memory device that is responsive to a voltage level in one of the motor windings during the time the bemf signal thereof is at approximately a zero reference, thereby being representative of the phase difference between the current signal of the winding and the bemf signal thereof. The resulting drive signal applied to the motor is such that the current provided to each winding is substantially in phase with the back electromotive force (bemf) signal corresponding to the winding. This results in the motor having a more optimal torque. Assuming that the bemf signals are sinusoidal or nearly sinusoidal, causing the current and bemf signal for each winding to be substantially in phase with each other results in the torque of the motor being substantially constant and ripple free.
The operation of the controller includes reading stored data representing the drive signal profile based in part upon the voltage level in one of the windings when the corresponding bemf signal is at a zero voltage. Drive signals are generated based upon the data read and applied to the windings of the polyphase motor so that the current applied to each winding is substantially in phase with the corresponding bemf signal.